(1) Field of the Invention
This invention relates to a circularly polarised microstrip antenna for microwaves.
(2 ) Prior Art
Short range mobile microwave links often use circularly polarised signals to minimise the effects of multipath interference. Because of their lightweight and flat profile, microstrip antennas are prime candidates for these applications. However, the axial ratio of conventional circularly polarised microstrip antennas deteriorates rapidly for scan angles greater than 30.degree. to 100.degree..
For many applications, the methods of achieving good axial ratio out to wide scan angles by empirically adjusting the ground plane size or by using a corrugated ground plane are not practical. Furthermore, while the bandwidth of a typical path antenna is adequate for the transmission of TV signals, it usually permits the antenna to be used for only one or two channels. The only method for adjusting the resonant frequency of a conventional patch antenna for a given substrate is to change the size of the patch.
For the substrates, the field patterns of a circular disc antenna can be calculated. For a conventional "Teflon" (registered trade mark) fibreglass substrate antenna (E.sub.r =2.55) the 3dB beamwidths of the E and H plane field patterns are 52.degree. and 40.degree.. If air is the substrate (E.sub.r =1) the E and H plane 3dB beamwidths are 30.degree. and 36.degree. respectively. These results suggest that for an intermediate dielectric constant substrate, the E and H plane 3dB beamwidths should be approximately equal. Experiments have shown this to be correct and it has been found that the axial ratio is much better at wide scan angles (i.e. greater than 40.degree. for a dielectric constant within the range of approximately 1.25 to 1.4 (i.e. 1.25 &lt;E.sub.r &lt;1.4). However metal clad substrates with dielectric constants in this range are not readily available although expensive honeycomb material may be obtained.